unvtop.tex (cf https://tex.stackexchange.com/a/463739/4686)

unvtop.tex

\documentclass{article}

\usepackage{showframe}     %% <- show boundary of text area
\usepackage{tikz}          %% <- To draw the red brackets
\usepackage{tikzpagenodes} %% <- to place them correctly

% variant from the \insertvbox of https://tex.stackexchange.com/a/463739/4686
\newcommand*\unvtop[1]{% #1 is a \vtop !
  \par
\begingroup
  \setbox2=\vbox{\unvcopy#1}%   %% <- box2 is a vbox
%
% cf TeX by Topic, explanations of TeX mechanism at bottom of page 186
%  (in 15.1 Interline glue) and taking into account in particular:
%     "the \prevdepth is not updated by an \unvbox or \unvcopy command"
% page 81, 5.7 Unboxing
%     Beware of the following subtlety: unboxing in vertical mode does not add
%     interline glue between the box contents and any preceding item. Also,
%     the value of \prevdepth is not changed,
%
  \dimen0=\dimexpr\baselineskip-\prevdepth-\ht#1\relax % #1 is a \vtop!
  \ifdim\dimen0<\lineskiplimit
     \vskip\lineskip
  \else
     \vskip\glueexpr\baselineskip-\dimexpr\baselineskip-\dimen0\relax\relax
  \fi
  \unvcopy#1% allows pagebreaks and in fact may recompute interline glue
  \prevdepth\dp2
\endgroup
}


\parskip=0pt   %% <- default value is "0pt plus 1pt"

\newcommand\redbracket{% %% <- for the little red brackets in the left margin
  \begin{tikzpicture}[remember picture, overlay,]
      \draw[red] (current page text area.north west) -- +(-2pt,0)
                   -- node[left]{\texttt{\detokenize{\topskip}}} +(-2pt,-\topskip) -- +(0,-\topskip);
      \foreach \X in {0, ..., 7} {
        \draw[red] ([yshift=-\topskip-\X\baselineskip]current page text area.north west) -- +(-2pt,0)
                   -- node[left]{\texttt{\detokenize{\baselineskip}}} +(-2pt,-\baselineskip) -- +(0,-\baselineskip);
      }
  \end{tikzpicture}%
}

\begin{document}

\newsavebox{\TitleBox}
\newsavebox{\BodyBox}
\setbox\TitleBox=\vtop{\noindent\textbf{A title that takes\\up more than\\two lines}}
\setbox\BodyBox=\vtop{\noindent Lorem\\ipsum\rule[-3pt]{1cm}{3pt}}

\vspace*{16.5cm}% testing breaking across pages

\unvtop\TitleBox

\unvtop\BodyBox

\unvtop\BodyBox

\unvtop\BodyBox

%\the\prevdepth

\redbracket


\clearpage

\noindent\textbf{A title that takes\\up more than\\two lines}

\noindent Lorem\\ipsum \par\noindent Lorem\\ipsum \par\noindent Lorem\\ipsum

\redbracket

\end{document}